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Presentation on theme: "The Origin of Species chapter 24."— Presentation transcript:

3 the origin of species The beginning of new forms of life.Speciation, key process.Explains, macroevolution, the origin of new taxonomic groups.Two patterns:1) anagenesis- linear evolution in which the entire population changes to be different from and to replace the ancestral population. (Lamarckian)2) cladogenesis- branching evolution that creates a greater diversity of sister organisms. Each branch is called a clade.

5 populations & species Populations are groups of individuals thatare the same specieslive in the same geographical area at the same time.A species is the largest unit of populationreproductively compatibleGene flow: possible to produce viable fertile offspringRegardless of geographical barriers

6 A species can be divided into subspecies, if they become reproductively isolated.Subspecies are different due to pre and/or postzygotic barriers:Prezygotic reproductive barriers:impede mating between species or hinder the fertilization of ova.b) Postzygotic reproductive barriers:prevent hybrid zygote from developing into a viable, fertile adult.If subspecies occur together, but remain reproductively isolated, these subspecies may eventually become two distinct species.

7 The biological species conceptis based on infertility rather than physical similarity. (horse + donkey = mule)

8 species species is Latin for “kind” or “appearance”biological species concept:Population or group of populationsmembers have the potential to interbreed with one another to produce viable, fertile offspringcannot produce viable, fertile offspring with members of other species.

10 Figure 24.5 A summary of reproductive barriers between closely related species

11 prezygotic barriers: Factors that lead to Reproductive IsolationEcographic Isolation: (geographic) isolationex. Asian and African elephantsB) Habitat Isolation: two species live in different habitats within the same area.ex. Garter snakes- one aquatic, one terrestrialC) Seasonal/Temporal Isolation: two species that breed during different times of the day, seasons, or years cannot mix their gametes.ex. Skunks: S. gracilis mates in late summer;S. putorius mates in late winter.

13 Figure 24.3 Courtship ritual as a behavioral barrier between species

14 Behavioral Isolation: signals to attract mates, elaborate behaviors, courtship rituals differ between species.ex. Eastern & Western Meadowlark songs differMechanical Isolation: anatomical incompatibility.ex. Insect copulatory organs don’t fit togetherfloral anatomy specialized to one pollinatorF) Gamete Isolation: incompatibility between sperm/egg.ex. Sperm of one species may not be able to survive in the environment of the female reproductive tract of another species.gamete recognition based on complementary molecules found on sperm/egg surfaces.

16 How do new species arise?1) By geographic isolation:This is the way the flora and fauna of the Galapagos Islands evolved.The barrier prevents gene flow.When two different species arise this way, it is called allopatric speciation.Greek: allos, other & patria, homeland

18 The factors that lead to divergence: A) size of population (small)Allopatric speciation of squirrelsin the Grand CanyonThe factors that lead to divergence:A) size of population (small)The founder effect- genetic drift attributed to colonization by a limited number of individuals from a parent population.B) ability of organism to move about (isolation)C) harshness/ differences of new environment.

19 A famous example of divergent evolution/speciation: Adaptive radiationAdapative radiation is evolution of many diversely adapted species from a common ancestor.Example: Darwin’s FinchesThe 14 species of Finch evolved from one species of ancestral finch.They have adapted to exploit different food sources with differently shaped beaks and feeding behaviors.They exhibit character displacement - evolutionary change driven by competition among species for a limited resource (eg. Food)Gause’s Law- competitive exclusion principle.

22 2) If two different species arise from a population without geographic barriers, it is called sympatric speciation.Examples of sympatric speciation: balanced polymorphism, polyploidy, hybridization.Polyploidy (having more than the diploid number of chromosomes) and chromosomal changeThis condition is common in plants and less common in animals.It can make offspring reproductively isolated from their parental species. (post-zygotic barrier is created in one generation)Polyploid population can self-pollinate, mate with other polyploids, or reproduce by asexual propagation.

25 Causes of Polyploidy:accidents during meiosis (autopolyploidy) results in the wrong number of sets of chromosomes in the gametesthe contribution of two different species to a polyploid hybrid (allopolyploidy) non-homologous chromosomes can’t align during meiosis.The chemical colchicine induces polyploidy.

26 summaryIn allopatric speciation, a new species forms while geographically isolated from its ancestor.Sympatric speciation requires the emergence of some type of reproductive barrier that isolates the gene pool of a subset of a population without geographic separation from the parent population.

27 PATTERNS OF EVOLUTIONDivergent Evolution- two or more species originate from a common ancestor. homologous traits.Convergent Evolution- two unrelated species that share similar traits. Arise not from a common ancestor but because each species has independently adapted to similar ecological conditions or lifestyles. analogous traits. Ex. Shark, porpoises, penguins bodiesEx. Eyes of squids and vertebrates.Parallel Evolution- two related species making similar evolutionary changes after their divergence. Ex. Marsupial and Placental mammals. analogous traits.Coevolution- tit-for-tat evolution of one species in response to new adaptation that appear in another species.ex. Pollinators-Flowering Plants

30 Punctuated Equilibrium (proposed by Stephen J. Gould)A catastrophic event or major genetic change occurs, rapid evolution and speciation occurs.The new population works back toward a long period of no evolution (few or no transitional forms.)The Cambrian Explosion represents a period in time(560 MYA) where we see diversification of animal phyla.

31 Patterns of macroevolutionPhyletic gradualism- evolution occurs by the gradual accumulation of small changes. The intermediate stages of evolution not represented by fossils merely testifies to the incompleteness of the fossil record.Punctuated Equilibrium- evolutionary history consists of geologically long periods of stasis with little or no evolution, interrupted or “punctuated” by geologically short periods of rapid evolution.